参考答案：

#include<iostream>

using namespace std;

const int N = 310;
int d[N][N];
int n, m, k;
const int INF = 0x3f3f3f3f;

void floyd()
{
    for (int k = 1; k <= n; ++k)
        for (int i = 1; i <= n; ++i)
            for (int j = 1; j <= n; ++j)
            {
                d[i][j] = min(d[i][j], d[i][k] + d[k][j]);
            }
}
int main()
{
    cin >> n >> m >> k;

    for (int i = 1; i <= n; ++i)
        for (int j = 1; j <= n; ++j)
        {
            if (i == j) d[i][j] = 0;
            else d[i][j] = INF;
        }

    while (m--)
    {
        int a, b, c;
        cin >> a >> b >> c;
        d[a][b] = min(d[a][b], c);
    }

    floyd();

    while (k--)
    {
        int a, b;
        cin >> a >> b;

        if (d[a][b] > INF / 2) cout << "impossible" << endl;
        else cout << d[a][b] << endl;
    }

    return 0;
}